Purification of tricresyl phosphate



United States Patent 2,716,658 PURIFICATION OF TRICRESYL PHOSPHATE Jacob Rosin, Maplewood, N. J., assignor to Montrose Chemical Company, a corporation of New Jersey No Drawing. Application August 30, 1952, Serial No. 307,373

3 Claims. (Cl. 260-461) This invention relates to a process for purifying tricresyl phosphate. When tricresyl phosphate is produced, the crude product ordinarily has a poor color and contains oxidizable substances, both of which lower its value. It has heretofore been the practice to purify tricresyl phosphate by treatment with an oxidizing agent such as potassium permanganate to improve the color and to remove substances susceptible to oxidation. This treatment, as it has been practiced in the past, has been expensive, both from the standpoint of the use of relatively costly reagents and the fact that the yield of tricresyl phosphate is reduced, since a portion of the tricresyl phosphate reacts with the potassium permanganate.

I have found that tricresyl phosphate ordinarily has only two types of impurities. The first of these is of an acidic nature and consists primarily of free cresol and cresol acid phosphates; this type of impurity may be readily removed from the reaction mixture by an ordinary alkaline wash. The second type of impurity consists of oxidation and/0r polymerization products in which the hydroxyl group has lost its acidic nature, which prevents the impurity from being washed out by an alkaline wash. 1 have found that the second type of impurity may be removed by treating crude tricresyl phosphate with an alkaline aqueous solution in the presence of an amphoteric metal such as zinc, tin, aluminum and the like and which reacts with the alkali to produce hydrogen in the nascent state. By such a treatment, the impurities are made alkali soluble and are thus removed in the alkaline wash.

Any caustic alkali may be employed in carrying out this process, but I prefer sodium hydroxide because of its low cost, ready availability and eflicient results. It is not necessary that a concentrated solution be used and I have found that a 1% to 3% aqueous solution of sodium hydroxide is entirely suitable. The treatment may be carried out at room temperature although higher temperatures may be preferred since the tricresyl phosphate is less viscous at the higher temperature and thus the alkaline solution is more easily mixed with it. Preferably, the metal is added in a very finely divided form such as in the form of a metal dust. The amount of metal which is necessary is not large and ordinarily about 1% by weight of the tricresyl phosphate is sufiicient. In many instances, a smaller amount can be used.

The quantity of the alkali solution which is used is not critical, but ordinarily about one volume of the alkaline solution per volume of the tricresyl phosphate will be found to be suitable. In carrying out the invention, it is only necessary to stir together the tricresyl phosphate, the aqueous alkaline solution, and the finely divided metal. Preferably, stirring is continued for about half an hour, the aqueous layer decanted and the alkaline wash repeated in presence of finely divided metal dust about three times, the duration of each wash being rather short, about 10-15 minutes. It is important not to follow up the caustic washes with a series of water washes intended to wash out the alkali entirely, since some hydrolysis occurring during this long procedure may yield phenolic material which would not go into the aqueous layer because the pH of the water layer is not high enough at this stage. As soon as the last alkaline wash does not yield phenolic matter on acidification the batch is acidified to prevent hydrolysis, and then neutralized with an excess of calcium carbonate or magnesium oxide. Instead of acidification followed by neutralization, one may treat the washed tricresyl phosphate with magnesium sulfate which will neutralize the alkali without rendering the solution acid. The neutral tricresyl phosphate is then dried by heating in a vacuum or by treating with a dehydrating agent, and filtered.

It has been found that this treatment is extremely eifective and when a ten gram sample of tricresyl phosphate, treated in accordance with the present invention, is shaken vigorously for two minutes with 50 ml. of freshly prepared 0.01 N potassium permanganate solution, the solution keeps the purple color unchanged for at least thirty minutes. Further, the color of the treated tricresyl phosphate is entirely satisfactory.

The following working example illustrates the present preferred method of carrying out my invention.

Equivalent molar quantities of phosphorous oxychloride and cresol are heated together in presence of .catalysts such as aluminum chloride, zinc chloride, etc., until there is no further evolution of HCl from the reaction mixture. The reaction mixture is then washed with warm water, followed by a caustic wash with one volume, based on the quantity of the reaction mixture, of 2% sodium hydroxide solution. There is added to this caustic mixture 1% by weight based on tricresyl phosphate of zinc dust, and the mixture is stirred and kept at a temperature of about 20 C. for thirty minutes. The aqueous layer is then decanted and the alkaline wash repeated in presence of the metal powder for about ten minutes. This procedure is repeated until the alkaline wash does not show any phenolic matter on acidification. Then the tricresyl phosphate is acidified by addition of diluted sulfuric or hydrochloric acid, and the mixture is then neutralized by the addition of calcium carbonate. The mixture is then filtered, and dried by heating in a vacuum to produce tricresyl phosphate having a satisfactory color and being substantially free from oxidizable substances.

I claim:

1. The process for treating crude tricresyl phosphate, consisting of washing the reaction mixture with a dilute aqueous alkaline solution in the presence of about 1% based on the weight of tricresyl phosphate of an amphoteric metal until the aqueous wash is free from phenolic materials, neutralizing the mixture, drying and filtering it.

2. The process of claim 1 wherein the alkalizing solution used contains from about 1% to about 3% of sodium hydroxide.

3. The process of claim 1 wherein is finely divided zinc.

the amphoteric metal References Cited in the file of this patent UNITED STATES PATENTS 2,365,121 Traylor Dec. 12, 1944 

1. THE PROCESS FOR TREATING CRUDE TRICRESYL PHOSPHATE CONSISTING OF WASHING THE REACTION MIXTURE WITH A DILUTE AQUEOUS ALKALINE SOLUTION IN THE PRESENCE OF ABOUT 1% BASED ON THE WEIGHT OF TRICRESYL PHOSPHATE OF AN AMPHOTERIC METAL UNTIL THE AQUEOUS WASH IS FREE FROM PHENOLIC MATERIALS, NEUTRALIZING THE MIXTURE, DRYING AND FILTERING IT. 